delphi - 如何在 Delphi 中根据 TList 所包含对象的任意属性对 TList 进行排序？

Question

我有一个 TList。它包含相同类型的对象的集合。这些对象是 TPersistent 的后代，并且具有大约 50 个不同的已发布属性。

在我的应用程序中，用户可以对这些对象进行搜索，搜索结果显示在 TDrawGrid 中，显示的特定列基于所搜索的属性。例如，如果用户搜索“发票”，则结果网格中会显示“发票”列。我希望能够让用户对这个网格进行排序。当然，更重要的是我不会预先知道网格中有哪些列。

通常要对 TList 进行排序，我只需创建一个函数，例如 SortOnName( p1, p2)，然后调用 TList 的 sort() 方法。我想更进一步，找到一种将属性名称传递给排序方法并使用 RTTI 进行比较的方法。

当然，我可以创建 50 种不同的排序方法并使用它们。或者，全局设置一个变量或作为执行所有这些工作的类的一部分，以向排序方法指示要排序的内容。但我很好奇是否有任何 Delphi 专业人士对于如何实现这一点有其他想法。

Answer 1

Delphi 7版本下面是如何实现这一目标的示例。我使用Delphi2010来实现它，但它至少应该在Delphi7中工作，因为我直接使用TypInfo单元。

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    ListBox1: TListBox;
    Edit1: TEdit;
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
    FList: TList;
    procedure DoSort(PropName: String);
    procedure DoDisplay(PropName: String);
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

uses
  TypInfo;

var
  PropertyName: String;

type
  TPerson = class
  private
    FName: String;
    FAge: Integer;
  published
  public
    constructor Create(Name: String; Age: Integer);
  published
    property Name: String read FName;
    property Age: Integer read FAge;
  end;

{ TPerson }

constructor TPerson.Create(Name: String; Age: Integer);
begin
  FName := Name;
  FAge := Age;
end;

function ComparePersonByPropertyName(P1, P2: Pointer): Integer;
var
  propValueP1, propValueP2: Variant;
begin
  propValueP1 := GetPropValue(P1, PropertyName, False);
  propValueP2 := GetPropValue(P2, PropertyName, False);

  if VarCompareValue(propValueP1, propValueP2) = vrEqual then begin
    Result :=  0;
  end else if VarCompareValue(propValueP1, propValueP2) = vrGreaterThan then begin
    Result :=  1;
  end else begin
    Result := -1;
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FList := TList.Create;
  FList.Add(TPerson.Create('Zed', 10));
  FList.Add(TPerson.Create('John', 20));
  FList.Add(TPerson.Create('Mike', 30));
  FList.Add(TPerson.Create('Paul', 40));
  FList.Add(TPerson.Create('Albert', 50));
  FList.Add(TPerson.Create('Barbara', 60));
  FList.Add(TPerson.Create('Christian', 70));

  Edit1.Text := 'Age';

  DoSort('Age'); // Sort by age
  DoDisplay('Age');
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  DoSort(Edit1.Text);
  DoDisplay(Edit1.Text);
end;

procedure TForm1.DoSort(PropName: String);
begin
  PropertyName := PropName;
  FList.Sort(ComparePersonByPropertyName);
end;

procedure TForm1.DoDisplay(PropName: String);
var
  i: Integer;
  strPropValue: String;
begin
  ListBox1.Items.Clear;

  for i := 0 to FList.Count - 1 do begin
    strPropValue := GetPropValue(FList[i], PropName, False);
    ListBox1.Items.Add(strPropValue);
  end;
end;

end.

顺便说一句，我使用了一个简单的表单，其中包含一个列表框、一个编辑和一个按钮。列表框显示已排序的列表 (FList) 的内容。该按钮用于根据用户在编辑框中输入的内容对列表进行排序。

Delphi 2010 版本(使用对方法的引用)

unit Unit2;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm2 = class(TForm)
    ListBox1: TListBox;
    Edit1: TEdit;
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
    FList: TList;
    FPropertyName: String; { << }
    procedure DoSort(PropName: String);
    procedure DoDisplay(PropName: String);
    function CompareObjectByPropertyName(P1, P2: Pointer): Integer; { << }
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

uses
  TypInfo;

type
  TPerson = class
  private
    FName: String;
    FAge: Integer;
  published
  public
    constructor Create(Name: String; Age: Integer);
  published
    property Name: String read FName;
    property Age: Integer read FAge;
  end;

{ TPerson }

constructor TPerson.Create(Name: String; Age: Integer);
begin
  FName := Name;
  FAge := Age;
end;

/// This version uses a method to do the sorting and therefore can use a field of the form,
/// no more ugly global variable.
/// See below (DoSort) if you want to get rid of the field also ;)
function TForm2.CompareObjectByPropertyName(P1, P2: Pointer): Integer; { << }
var
  propValueP1, propValueP2: Variant;
begin
  propValueP1 := GetPropValue(P1, FPropertyName, False);
  propValueP2 := GetPropValue(P2, FPropertyName, False);

  if VarCompareValue(propValueP1, propValueP2) = vrEqual then begin
    Result :=  0;
  end else if VarCompareValue(propValueP1, propValueP2) = vrGreaterThan then begin
    Result :=  1;
  end else begin
    Result := -1;
  end;
end;

procedure TForm2.FormCreate(Sender: TObject);
begin
  FList := TList.Create;
  FList.Add(TPerson.Create('Zed', 10));
  FList.Add(TPerson.Create('John', 20));
  FList.Add(TPerson.Create('Mike', 30));
  FList.Add(TPerson.Create('Paul', 40));
  FList.Add(TPerson.Create('Albert', 50));
  FList.Add(TPerson.Create('Barbara', 60));
  FList.Add(TPerson.Create('Christian', 70));

  Edit1.Text := 'Age';

  DoSort('Age'); // Sort by age
  DoDisplay('Age');
end;

procedure TForm2.Button1Click(Sender: TObject);
begin
  DoSort(Edit1.Text);
  DoDisplay(Edit1.Text);
end;

procedure TForm2.DoSort(PropName: String);
begin
  FPropertyName := PropName; { << }
  FList.SortList(CompareObjectByPropertyName); { << }

  /// The code above could be written with a lambda, and without CompareObjectByPropertyName
  /// using FPropertyName, and by using a closure thus referring to PropName directly.

  /// Below is the equivalent code that doesn't make use of FPropertyName. The code below
  /// could be commented out completely and just is there to show an alternative approach.
  FList.SortList(
    function (P1, P2: Pointer): Integer
    var
      propValueP1, propValueP2: Variant;
    begin
      propValueP1 := GetPropValue(P1, PropName, False);
      propValueP2 := GetPropValue(P2, PropName, False);

      if VarCompareValue(propValueP1, propValueP2) = vrEqual then begin
        Result :=  0;
      end else if VarCompareValue(propValueP1, propValueP2) = vrGreaterThan then begin
        Result :=  1;
      end else begin
        Result := -1; /// This is a catch anything else, even if the values cannot be compared
      end;
    end);
  /// Inline anonymous functions (lambdas) make the code less readable but
  /// have the advantage of "capturing" local variables (creating a closure)
end;

procedure TForm2.DoDisplay(PropName: String);
var
  i: Integer;
  strPropValue: String;
begin
  ListBox1.Items.Clear;

  for i := 0 to FList.Count - 1 do begin
    strPropValue := GetPropValue(FList[i], PropName, False);
    ListBox1.Items.Add(strPropValue);
  end;
end;

end.

我用{ << }标记主要变化。